Positronium-mediated method for identifying a contaminant gas in a gaseous mixture

ABSTRACT

The present invention discloses an in vitro method to identify a contaminant gas in a specimen comprising a mixture of gases as a function of the decay rate of at least one species of positronium. The positronium is obtained by directing the positrons from a positron source in to a vessel that contains a specimen containing the mixture of gases comprising a contaminant gas to be identified.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional application of U.S. application Ser.No. 10/342,063 filed on Jan. 14, 2003, now U.S. Pat. No. 7,041,508,which itself claimed priority from now abandoned Provisional PatentApplication Ser. No. 60/354,840, filed Feb. 6, 2002, and entitledPOSITRON AIR MONITOR (PAM).

FIELD OF THE INVENTION

The present invention generally relates to hazardous material sensors,and more particularly to sensors for use in monitoring the level ofhazardous contaminants in the ambient environment.

BACKGROUND OF THE INVENTION

Approximately five and three quarter million shipping containers a year,or ninety-five percent of all international origin goods, arrive in theU.S. by sea. At U.S. ports, inspecting a 20 to 40 foot long shippingcontainer can take four customs inspectors about four hours. At thatrate, often fewer than two percent are opened for inspection, and thegreat majority never pass through any existing sensors, e.g., x-raymachines, gamma-ray probes, or the like. In addition, U.S. borders tothe north and south accommodate 125 million vehicles (including 11.2million trucks), 2.2 million rail cars and 500 million people on anannual basis. Significantly, such prior art sensors are essentiallyblind to biological hazards. Front line inspectors, customs officers andother law enforcement officers have consequently called for newtechnology to screen shipments, whether from air, ship or rail, so thattime consuming and labor intensive searches can be minimized anddangerous cargo can be prevented from entering the U.S.

Other applications include First Responders (EMT's, fire departments,and law enforcement agencies) that would benefit from an all-inclusivedevice to assess contaminated areas or detect hazardous materials. Inaddition, the device could be used for the inspection of air handlingsystems for “sick building syndrome” or Legionaries' disease and toxicmold in commercial and residential buildings.

There are many substances which have very small vapor pressures, butwhose presence in air is nonetheless undesirable because they are verytoxic or indicate the presence of unwanted substances hazardouschemicals, biological agents, explosives, drugs, etc. (hereinafterreferred to as “BCA's” or Biological, Chemical Agents). Currentdetection methods for BCA hazards are extremely slow, and are oftenbased upon complicated chemical or mechanical concepts, use ofmulti-step and labor intensive approaches, or the need for replaceablesupplies (consumables). In addition, such prior art devices are oftenvery large (not portable) and expensive.

The saturation concentrations of these hazardous substances in air atroom temperature suggest that they can be detected using existingtechniques. However, in the real world, they are unlikely to bepresented to a detector with a sufficient volume of saturated air tomake such detection easy. At best, the fraction of molecules availableto a ‘sniffer’ will be reduced by a few orders of magnitude. Therefore,sensors must be able to detect these materials at vapor concentrations afew orders of magnitude less than their saturation concentrations.

It is well known in the art to use the anti-electron (commonly referredto as a “positron”) to probe the structure of molecules. This field owesmost of its existence to the study of the crystalline structure ofsemiconductor materials and the structure of polymers, e.g., isolationof irregularities in semiconductors and polymers. It is known thatpositrons of cosmic origin annihilate with extremely dilute moleculargases in interstellar space. Gamma rays that have been captured andrecorded by satellites orbiting the Earth provide evidence for theexistence of gases in incredibly small concentrations, and can evendistinguish among various species of molecules. This technology has beenfurther explored academically, for example, by K. Iwata et al., in theirpublication entitled: “Measurements of positron-annihilation rates onmolecules”, Physical Review A 51, 473, 1995, which publication is herebyincorporated herein by reference.

The foregoing positron annihilation method generally comprises a processin which a positron is injected into physical matter from a positronsource, and the lifetime of the positron (i.e., the time betweeninjection and annihilation) is measured to indirectly determine variouscharacteristics of the matter. A positron is the anti-particle of anelectron, and is an elementary particle having the same mass and theopposite charge as an electron. When positrons are implanted in a solidthey are rapidly thermalized and annihilate with electrons. It is knownthat a positron and an electron briefly form an electron-positron pair(via coulomb forces) when the two particles meet in a molecular crystalor in an amorphous solid material, and then the pair annihilates. Thepositron-electron pair behaves in a manner similar to a particle in abound state, and is referred to as “positronium.”

When positronium annihilates, two or three annihilation gamma-rays areemitted. There are two types of positronium, para-positronium andortho-positronium. The spins of the electron and the positron areanti-parallel in the para-positronium and parallel in theortho-positronium. Para-positronium decays into two 511 kiloelectronvolt(keV) gamma rays, one in each of two directions with an angel of 180°between them. Ortho-positronium decays into three gamma rays, the sumenergy of which is 1022 keV. While the lifetime of a para-positroniumpair is about 0.13 nanoseconds (ns), the lifetime of anortho-positronium pair depends upon the electron density in thesurroundings of the positronium. The mean lifetime of ortho-positroniumin vacuum is about 140 ns, when it is annihilated in a self-annihilationprocess. However, the lifetime decreases down to the range from about 1to about 5 ns when an ortho-positronium pair annihilates through a“pick-off” process in which the positronium takes electrons from thesurrounding matter. With the aforementioned positron annihilationmethod, a positron lifetime is determined by measuring the timevariation in intensity of the annihilation gamma-rays emanating from thematerial into which the positrons had been injected.

The use of ortho-positronium decay is known for the determination of thelocation and size of crystal lattice defects. For example, whenortho-positronium exists in a vacancy-type defect, the measured lifetimeof the ortho-positronium correlates well with the size of the defect.With increases in the size of the vacancy-type defect, the probabilitythat the ortho-positronium will succumb to “pick-off” annihilation withan electron oozed out from the inner wall of the defect decreases,resulting in longer lifetimes of the ortho-positronium. Thus, the sizeof the defect can be determined by measuring the lifetime of theortho-positronium. It is also known, however, that the lifetime ofortho-positronium tends to saturate when the radius of the defectincreases beyond a certain value, e.g., about 0.5 nanometers (nm) sothat the maximum value of the radius of a defect measurable by thismethod is about 0.5 nm.

There is a need in the art for an improved method and apparatus forsensing and monitoring the ingress of so called hazardous BCA materialsinto the United States of America. It would be of benefit if theforegoing positron annihilation method could be used to detect suchhazardous BCA materials.

SUMMARY OF THE INVENTION

The present invention provides a device for measuring the presence of asmall concentration of at least one hazardous material within a vessel.A positron source emits positrons into an annihilation region of thevessel that is spaced apart from the positron source. A plurality ofspecies of positronium are formed from the positrons as they interactwith a sample of the ambient environment, e.g., an ambient air sample,disposed within the vessel. The annihilation region within the vessel ispositioned such that at least a portion of the sample to be monitoredmust pass in annihilation proximity of the positrons so as to form atleast one of the species of positronium. Two gamma ray detectors arelocated externally of the vessel, and shielded from the positron source,for detecting gamma rays generated primarily by the absorption of thespecies of positronium within the annihilation region.

In another embodiment of the invention, a device for measuring theconstituents of a gaseous sample is provided that includes a source ofpositrons which positrons are deposited within an annihilation region ofa vessel. The annihilation region is spaced apart from the positronsource and also contains the gaseous sample. A plurality of positroniumspecies are formed through interaction of the positrons with the gaseoussample. Two gamma ray detectors are disposed adjacent to theannihilation region for detecting gamma rays generated by the decay ofthe species of positronium within the annihilation region.

A method for detecting contaminants in a specimen containing at leastone known gas is also provided where a source of positrons is arrangedso as to direct positrons into a vessel containing a specimen of the atleast one known gas and at least one contaminant so as to form aplurality of species of positronium. The timing of the application ofthe positrons to the vessel is sensed along with the annihilation ofeach of the plurality of species of positronium in the vessel. The timedelay between the time of application of each positron to the vessel andthe annihilation of at least one species of positronium is measured toobtain a decay rate characteristic of the specimen of gas, each timedelay being measured over a substantial time scale, the time scale fordetermining each time delay being in the range from about 1 ns to about15 ns. The contaminants within the specimen of gas are then determinedas a function of the decay rate of the at least one species ofpositronium.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will bemore fully disclosed in, or rendered obvious by, the following detaileddescription of the preferred embodiments of the invention, which are tobe considered together with the accompanying drawings wherein likenumbers refer to like parts and further wherein:

FIG. 1 is a schematic perspective view of an apparatus formed inaccordance with the present invention for measuring the presence of asmall concentration of at least one hazardous material in a sample ofambient atmosphere;

FIG. 2 is a schematic perspective view, similar to FIG. 1, having thesignal processing and storing systems removed for clarity ofillustration;

FIG. 3 is a stylized illustration of various molecular and biologicalstructures that may be present within an annihilation region of a vesselportion of the present invention;

FIG. 4 is a stylized illustration of a hazardous material moleculehaving species of positronium formed in association with atomicstructures of the molecule;

FIG. 5 is a stylized illustration of a para-positroniumelectron-positron pair;

FIG. 6 is a stylized illustration of an ortho-positronium pair;

FIG. 7 is a stylized illustration of a bacterium having a species ofpositronium formed with a portion of its molecular structure;

FIG. 8 is a graphic representation of a positron-lifetime curve of thetype formed by the method of the present invention;

FIGS. 9 and 10 are graphic representations of signature-curves of thetype resulting from the practice of the present invention; and

FIG. 11 is a graphic illustration of two signature curves, one for puremethanol and one for methanol with a small admixture of HTMPO.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This description of preferred embodiments is intended to be read inconnection with the accompanying drawings, which are to be consideredpart of the entire written description of this invention. The drawingfigures are not necessarily to scale and certain features and atomicstructures formed by the method of the invention may be shown highlyexaggerated in scale or in somewhat schematic form in the interest ofclarity and conciseness. In the description, relative terms such as“horizontal,” “vertical,” “up,” “down,” “top” and “bottom” as well asderivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,”etc.) should be construed to refer to the orientation as then describedor as shown in the drawing figure under discussion. These relative termsare for convenience of description and normally are not intended torequire a particular orientation. Terms including “inwardly” versus“outwardly,” “longitudinal” versus “lateral” and the like are to beinterpreted relative to one another or relative to an axis ofelongation, or an axis or center of rotation, as appropriate. Termsconcerning attachments, coupling and the like, such as “connected” and“interconnected,” refer to a relationship wherein structures are securedor attached to one another either directly or indirectly throughintervening structures, as well as both movable or rigid attachments orrelationships, unless expressly described otherwise. The term“operatively connected” is such an attachment, coupling or connectionthat allows the pertinent structures to operate as intended by virtue ofthat relationship. In the claims, means-plus-function clauses areintended to cover the structures described, suggested, or renderedobvious by the written description or drawings for performing therecited function, including not only structural equivalents but alsoequivalent structures.

Referring to FIGS. 1, 2, and 3, the present invention provides an airmonitor 5 and method for monitoring BCA contaminants 8 in a specimen ofambient air 10. It should be noted that a positron-based detectiondevice differs from most radiometric monitoring systems, such as alpharay and beta particle systems, since it measures the absorption ofpositrons which characteristically are associated with the generation ofa pair of gamma rays. Thus, while positrons are very sensitive tochanges in the media through which they flow, the gamma-ray photonswhich are generated from an annihilation event have good penetratingproperties and may be readily detected through relatively thick walls.

More particularly, air monitor 5 generally comprises an air measurementvessel 12, a positron source 18, gamma-ray detectors 20, and a signalprocessing and storing system 22. Measurement vessel 12 defines aninterior chamber 25, and includes an entrance port 27 and an exit port30 that are formed in the wall of measurement vessel 12 so that ambientair sample 10 (FIG. 3) may be deposited within the vessel for analysis.A variety of metals, ceramics, and polymers that are suitable formaintaining a partial vacuum may be used to form vessel 12.

Positron source 18 is preferably a ten curie (Ci) source whose ionizingradiation output is within present and future federally mandated limitsfor safe handling, e.g., Na₂₂ with a 2.6 yr. half-life. Of course, othersources of positrons, e.g., Ti₄₄/Sc₄₄ (49 yr.), Co₅₈ (70.8 day), orGe₆₈/Ga₆₈ (271 day), may also be used in connection with the presentinvention. Positron source 18 is typically housed in a container 32 thatallows for appropriate shielding so that the majority of positrons arereleased from an exit port 33 into a transfer conduit 34. It will beunderstood that in order to precisely monitor the annihilation events,gamma ray detectors 20 should be collimated to exclude radiation frompositron source 18, since most positron sources also emit gamma rays. Anannihilation region 35 within measurement vessel 12 is spaced apart frompositron source 18. Positrons 38 (identified by reference character“e+”) are generated by positron source 18 and directed through transferconduit 34 so as to be transferred into annihilation region 35 ofinterior chamber 25 by, e.g., an electrostatic or magnetic lens assembly40 (FIG. 2). Of course, it will be understood that positron source 18may be housed within measurement vessel 12 so that positrons 38 simplyenter annihilation region 35 as a result of natural decay and scatteringprocesses.

Electrostatic or magnetic lens assembly 40 is adapted to be sealinglymounted to positron source 18, via exit port 33, and may take severalforms. For example, electrostatic or magnetic lens assembly 40 maycomprise a plurality coaxially aligned, cylindrical tubes 43 that areformed from a highly conductive metal, e.g., copper or its alloys, orhighly magnetic metals, e.g., SmCo, NdFeB or the like. For anelectrostatic version of lens assembly 40, tubes 43 are individuallyinterconnected to a source of variable high voltage electrical potential44 in a manner well known to those of ordinary skill in the art (e.g.,cables 45). For a magnetic version of lens assembly 40, one or moretubes 43 would comprise a permanent magnet. In either case, tubes 43 aresized so as to fit within transfer conduit 34 arranged between positronsource 18 and interior chamber 25. For an electrostatic version of lensassembly 40, gaps 47 are defined between predetermined groups of tubes43 so as to form strong electric field gradients adjacent to the edgeportions of the tubes that are positioned on either side of a gap 47.Electrostatic or magnetic lens assembly 40 normally does not extend intomeasurement vessel 12, although it may do so, as needed, for aparticular design purpose. Both transfer conduit 34 and interior chamber25 are maintained at a similar partially evacuated state, i.e., internalpressure, as positron source 18.

A plurality of species of positronium are formed from the interaction ofpositrons 38 with at least BCA contaminants 8 in ambient air sample 10.In particular, at least a population of para-positronium 50(anti-parallel spins) and ortho-positronium 52 (parallel spins) areformed within measurement vessel 12 in which an electron 53 from amolecule of an BCA contaminant 8, e.g., a molecule of a hazardousmaterial or a molecule that is resident within the cell wall of a livingpathogenic organism, is paired with a positron 38 (FIGS. 4-7).Annihilation region 35 is located within interior chamber 25, andpositioned such that at least a portion of the sample of ambient air 10to be examined must pass in positron capture proximity of at least onepositron 38.

Referring again to FIGS. 1 and 2, gamma ray detectors 20 are externallylocated relative to measurement vessel 12, and are often shielded frompositron source 18. Gamma ray detectors 20 sense 511 keV gamma rays 67generated by the absorption of the species of positronium withinannihilation region 35. In a particularly advantageous embodiment of thepresent invention, gamma-ray detectors 20 comprise an array ofphotodetectors consisting of scintillator crystals 55 coupled tophotomultiplier tubes 60 (PMTs). When a photon strikes a detector 20, itproduces light in one of scintillator crystals 55 that is then sensed byPMT 60, which registers the event (count) by passing an electronicsignal to reconstruction processing circuitry comprising a pre-amplifier62 and a multichannel analyzer 63. The counts are stored in a database,and may be displayed on a conventional computer controlled displaymonitor 65. Scintillator crystals 55 must have certain properties, amongwhich are (1) good stopping power, (2) high light yield, and (3) fastdecay time. Stopping power is the ability to stop the 511 keV gamma-rayphotons 67 (FIG. 3) in as little material as possible so as to reducethe overall size of the photodetector, of which the scintillatorcrystals form a substantial portion.

It has been found that scintillator crystals formed from barium fluoride(BaF₂₎) are particularly well suited to the present invention. The useof BaF₂ as a scintillator material is described in Allemand et al., U.S.Pat. No. 4,510,394, which patent is incorporated herein by reference.BaF₂ emits light having two components: a slow component having a decayconstant of approximately 620 ns and a fast component having a decayconstant of approximately 0.6 ns. The fast component of BaF₂ emits lightin the ultraviolet region of the spectrum. Glass photomultiplier tubesare often transparent to ultraviolet light, so a quartz photomultipliertube is preferred for detecting the fast component of BaF₂. The fastcomponent gives BaF₂ very good timing resolution. Since BaF₂ is nothygroscopic, it is quite suitable for use in many high humiditylocations, e.g., seaports, outdoors, etc. The stopping power of BaF₂with respect to gamma-ray photons is higher than that of other wellknown scintillator materials, e.g., sodium iodide and cesium iodide.Moreover, BaF₂ is substantially insensitive to water, as well as tonumerous organic solvents such as ethanol, ethyl ether, acetone andmethanol. In addition, it can be easily machined or worked, e.g.compared with glass which is harder, but it is still not fragile.

Referring now to FIGS. 3 and 8-11, the inverse lifetime or natural widthΓ (decay rate expressed in decays per second), of a positron in a gas isgenerally expressed as: Γ=πr_(o) ²cnZ_(eff), where r_(o) and c areconstants, and are the classical electron radius and speed of light,respectively, n is the gas density, and Z_(eff) is the “effective”atomic number of the gas molecule. Thus, the physics of the process ofthe present invention is embodied in one number, Z_(eff). Significantly,Γ is not proportional to the atomic number of a molecule. It has beenfound that large molecules exposed to positrons generate annihilationsat a relatively faster rate than small molecules. In other words,Z_(eff) grows more rapidly than Z, especially as Z gets large. The decayrate may grow by about a factor of one thousand with every advance ofthe atomic number by a factor of ten.

For example, assuming that the decay rate for positrons 38 exposed tohydrogen molecules (Z=2) is one (in arbitrary units), then the decayrate in the presence of anthracene (C₁₄H₁₀, Z=94) is about four million.By calibrating the present invention for Z_(eff) for the BCA's inquestion, they may be detected in ambient air sample 10. Hence, thelifetime of complex molecular structures, e.g., the cell wall 65 of abacterium 68 that is itself built up from many complex molecularstructures, including proteins 69 and DNA 70, etc., each with a uniqueZ_(eff), are expected to be very short. On average, the lifetimes ofpositrons that are paired with complex molecular structures is oftenapproximately several thousands of times less than the Z_(eff) formolecular oxygen and nitrogen, which are the primary constituents ofambient air sample 10). By selection and measurement of a characteristicslope in a lifetime distribution, air monitor 5 allows for theidentification of macromolecules, and their different species, from thebackground of annihilations on light atoms and molecules normally foundin air. These light atoms and molecules include oxygen, nitrogen, inertgases, nitrous and carbon oxides from automobiles, power plants,airplanes and other fossil fuel burning machines, pesticides, naturallyand artificially occurring dusts, etc.

The average lifetime of each positron species 50,52 is measured byobserving and recording the positron annihilation generated gamma rays67 with gamma-ray detectors 20. A positron-lifetime curve 71 is createdbased on annihilation gamma rays 67, after accumulating tens ofthousands of counts over time (FIG. 8). Advantageously, when the countsare graphed as a function of time, a unique signature-curve 72 isrevealed for each particular molecular constituent in the sample ofambient air 10 (FIGS. 8-10). The slope of each signature-curve 72 a, 72b, 72 c, etc., is a measure of the mean lifetime, or age, of theortho-positronium state formed with an electron 53 from a givenmolecular constituent (FIG. 9). Computer system and display 65 of thetype well known in the art may be employed to de-convolute the aggregatepositron age curves 72 a-c into their constituent parts. A data base isprovided that contains an extensive library of individual BCAsignature-curves 80 a, 80 b, 80 c, etc., as well as signature-curves forcommon background distractions, e.g., commonly found molecularconstituents of ambient air at a particular location (FIG. 10).Signature-curves 72 are then compared to the BCA signature-curves 80 soas to determine the type and quantity of BCA molecules 8 in ambient airsample 10.

It should be understood that the positron age method is a physicalcharacterization of the molecular mass, and hence size, of a chemical orbiological agent. It is not a chemical characterization, such as wouldbe developed by observing the fluorescence of molecules underirradiation with laser light. In this sense, it is similar in output tothe well known mass spectrometer method where the device renders thecharge to mass ratio (q/m) of the sample. However, since the positronage measurement performed by air monitor 5 does not need todifferentiate between different charge states of a molecule, it isintrinsically simpler than a mass spectrometer.

The present invention may be employed to identify extremely smallconcentrations of chemical and/or biological agents in air. Referring toFIG. 11, upper curve 90 shows the results (counts per unit time) forpure methanol, (atomic number “Z”=18; and lower curve 92 shows theresults for pure methanol with a very small admixture of HTMPO, (Z=65).The separation of the two curves and their different slopes isindicative of the improved sensitivity for identifying a uniquemolecular species when practicing the present invention, that iscomparable to, or better than prior art techniques at less than a fewparts per million concentrations of BCA's. It is important to note, forexample, that the chemical composition of four nerve agents (Tabun,Sarin, Soman and VX) have marked similarities to HTMPO. Both containsignificant C-H and CH₃-N bonds. These nerve agents are somewhat heavier(average molecular wt. ˜180, vs. 112 for HTMPO) due to phosphorous andfluorine bonds, and thus display unique positron annihilation lifetimesignature-curves 80.

In operation, samples of air 10 are exposed to positrons 38 inmeasurement vessel 12. Including a 20% detector geometrical acceptancefor each of two gamma ray detectors 20, the total counting rate would be3.7×10⁵ (10 microCi) decays/sec×2×0.2=148 kHz. These events are almostexclusively the two gamma ray decay mode of para-positronium 50 states,constituting a sharp peak for times less than ˜2 ns. These events areconsidered background in detector 20. Including an efficiency of 1% forconversion of positrons into long-lived ortho-positronium 52 states,this gives a counting rate of 3.7×105 decays/sec×2×0.2×0.01=1.48 kHz.These events are the signal events (at times greater than ˜2 ns), fromwhich the desired lifetime measurement is determined.

In nuclear counting situations, background rates must be considered. Itis well known that the main source of background radiation comes fromfree or para-positronium annihilations on the walls of measurementvessel 12 or the gas itself, resulting in two back-to-back gamma rays 67of 511 KeV energy each. Recorded times for primary gamma rays 67 fromsuch background are often well under a few nanoseconds (ns). Someprimary gamma-rays 67 will scatter off electrons in the walls of vessel12 or the gas making up most of ambient air sample 10, and “bounce”around the interior chamber 25 before annihilating, resulting in delayedtimes. If, for example, the dimensions of interior chamber 25 areapproximately 10 cm on average, and the speed of gamma rays is 3×10¹⁰cm/second, then one traversal of interior chamber 25 takes only about0.33 ns. Hence, a delay of 10 ns would require 33 scatters to achieve.

It is to be understood that the present invention is by no means limitedonly to the particular constructions herein disclosed and shown in thedrawings, but also comprises any modifications or equivalents within thescope of the claims.

1. A method for identifying contaminants in a specimen containing atleast one known gas comprising: positioning a source of positronsoutside of a vessel containing a mixture of at least one known gas andat least one contaminant; directing said positrons from said source intosaid vessel so as to form at least one species of positronium in saidvessel by an interaction of said positrons with said at least one knowngas and said at least one contaminant; sensing the timing of theapplication of said directed positrons to said vessel; sensing theannihilation of each of said at least one species of positronium in saidvessel; measuring the time delay between the time of application of eachpositron to said vessel and the annihilation of at least one species ofpositronium to obtain a decay rate characteristic of the specimen ofgas, each time delay being measured over a substantial time scale, thetime scale for determining each time delay being in the range from about1 to 5 ns; and identifying said at least one unknown contaminant withinsaid specimen of gas as a function of the decay rate of said at leastone species of positronium by correlating with known decay rates storedin a database.
 2. A method for identifying contaminants in a specimencontaining at least one known gas according to claim 1 furthercomprising shielding all but an exit portion of said source ofpositrons, thereby preventing gamma rays emanating from said source fromentering said vessel.
 3. A method for identifying contaminants in aspecimen containing at least one known gas according to claim 2 furthercomprising applying an electric field or a magnetic field to positronsexiting said positron source so as to direct said positrons entry intosaid vessel.
 4. A method for identifying contaminants in a specimencontaining at least one known gas according to claim 1 furthercomprising positioning at least two gamma-ray detectors adjacent to saidvessel so as to sense the annihilation of each of said at least onespecies of positronium in said vessel.
 5. A method for identifyingcontaminants in a specimen containing at least one known gas accordingto claim 1 further comprising forming an annihilation region within aninterior chamber of said vessel.
 6. A method for identifyingcontaminants in a specimen containing at least one known gas accordingto claim 5 further comprising directing said positrons from saidpositron source through a transfer conduit into said annihilation regionby applying an electric field or a magnetic field to positrons exitingsaid positron source prior to their entry into said vessel.
 7. A methodfor identifying contaminants in a specimen containing at least one knowngas according to claim 5 further comprising positioning said mixture iswithin said annihilation region such that at least a portion of saidmixture passes in positron capture proximity of at least one positron.8. A method for identifying contaminants in a specimen containing atleast one known gas according to claim 5 further comprising positioningat least one gamma ray detector in spaced relation to said annihilationregion, and shielding said positron source.
 9. A method for identifyingcontaminants in a specimen containing at least one known gas accordingto claim 1 further comprising observing and recording positronannihilation generated gamma rays with at least one gamma-ray detector.10. A method for identifying contaminants in a specimen containing atleast one known gas according to claim 5 further comprising forming apopulation of ortho-positronium within said annihilation region from anelectron from a molecule of said mixture that is paired with one of saidpositrons.
 11. A method for identifying contaminants in a specimencontaining at least one known gas according to claim 5 furthercomprising forming a population of ortho-positronium within saidannihilation region from an electron from a molecule comprising aportion of a biological or chemical material.
 12. A method foridentifying contaminants in a specimen containing at least one known gasaccording to claim 5 further comprising forming a population ofortho-positronium within said annihilation region from an electron froma molecule comprising a portion of an organism.
 13. A method foridentifying contaminants in a specimen containing at least one known gasaccording to claim 5 further comprising forming a population ofortho-positronium within said annihilation region from an electron froma portion of an anthrax spore.
 14. A method for identifying contaminantsin a specimen containing at least one known gas according to claim 5further comprising forming a population of ortho-positronium within saidannihilation region from an electron from a molecule comprising aportion of a nerve agent.
 15. A method for identifying contaminants in aspecimen containing at least one known gas according to claim 5 furthercomprising forming a population of ortho-positronium within saidannihilation region from an electron from a molecule comprising aportion of a chemical warfare agent.